1. Field of the Invention
The present invention relates to a scanning receiver for receiving a signal by scanning the frequency of a received signal, more particularly to a scanning receiver having a high speed scanning operation. The present invention further relates to a scanning method for scanning the frequency of a received signal in the scanning receiver.
2. Description of the Prior Art
FIG. 1 schematically represents an arrangement of a conventional scanning receiver. In this receiver, a signal received via an antenna 1 by an RF (radio frequency) circuit 2, is mixed with an oscillating frequency signal supplied from a frequency synthesizer (now shown.) in the RF circuit 2, and then converted into an intermediate frequency signal. The frequency synthesizer includes a PLL (phase-looked loop) circuit. Upon supply of reception frequency data from a microprocessor 4, the PLL circuit is locked up, so that this frequency synthesizer produces the oscillating frequency signal having a predetermined oscillating frequency. The intermediate frequency signal is supplied to an IF (intermediate frequency) circuit 6. A noise detecting circuit 8 detects noises contained in the intermediate frequency signal and changes the level of noise into a DC (direct current) voltage. When the resulting DC voltage obtained after the noise detecting is lower than a threshold value, the noise detecting circuit 8 knows that an audio signal has been received and generates an SC (Scan Control) signal. The SC signal is then supplied to the microprocessor 4. The microprocessor 4 interrupts the scanning operation, and the intermediate frequency signal derived from the IF circuit 6 is supplied to an AF (audio frequency) circuit 10. In the AF circuit 10, the intermediate frequency signal is converted into an AF signal by a frequency discriminator (now shown) and then is supplied to a speaker 11.
As described above, in the conventional scanning receiver, when the SC signal is generated in the noise detecting circuit 8, the scanning operation is interrupted or stopped. As a result, even when unwanted signals such as non-modulated signals and digital signals are received, the SC signal is generated in the noise detecting circuit 8 so that the scanning operation in the microprocessor 4 is stopped. This means that no discrimination can be established between audio signals and said unwanted signals. Under such circumstances, there is a problem that even when either a non-modulated signal or a digital signal is received, such unwanted signal may be erroneously recognized as an audio signal, thereby interrupting the scanning operation. This annoys users for the scanning receiver and makes the speed of a scanning operation low.